1. Field of the Invention
The present invention relates to a display device, particularly to a flat display device having an optical filter such as a polarizing plate.
2. Description of the Prior Art
In a conventional flat display device such as a liquid crystal display (LCD) device as described in Japanese Unexamined Patent Publication No. 2000-250062, a transparent electrode layer is provided on a polarizing plate formed on a color filter substrate of an LCD panel. The transparent electrode layer prevents the accumulation of static electricity in the LCD panel to avoid such trouble as flicker in a display screen. The static electricity is released to ground through a contact pin or contact finger or the like connected to the transparent electrode layer. In view of cost reduction, the transparent electrode layer may be removed, and the polarizing plate can be replaced by an electrically conductive polarizing plate.
An example of such conductive polarizing plate is disclosed in Japanese Unexamined Patent Publication No. 2000-085068. According to this document, in the conductive polarizing plate, a coat layer made of a transparent electrically conductive polymer, which contains at least acrylic ester having a quaternary ammonium salt group as a monomer component, is provided on the surface of a protective layer of a polarizer. Furthermore, a transparent protective film is provided on the surface of this coat layer. Thus, the conductive polarizing plate is configured.
FIG. 1 is a cross-sectional view showing an example of an LCD device in which an electrically conductive polarizing plate as described above is used. An LCD panel 100 includes two glass substrates, which are a thin-film transistor (TFT) substrate 101 and a color filter substrate 102, and a liquid crystal layer (not shown) interposed between these substrates. On surfaces of the two glass substrates, a TFT-side polarizing plate 104 and an electrically conductive polarizing plate 103 are respectively provided. Both sides of the LCD panel 100 are held between a front frame 107 and a chassis 105. The front frame 107 and the chassis 105 are made of an electrically conductive material. The conductive polarizing plate 103 and the front frame 107 are brought into electrical contact with each other through a contact finger 106.
An electrically conductive transparent electrode layer provided on the surface of the glass substrate at the side of the color filter has a surface resistance of 200Ω to 300Ω. Meanwhile, the conductive polarizing plate has a very large surface resistance of 30 MΩ to 500 MΩ. Accordingly, in a case where the conductive polarizing plate 103 as shown in FIG. 1 is used in an LCD device, electrical continuity between the conductive polarizing plate and the front frame 107 is ensured by pushing the conductive polarizing plate 103 with a force of not less than 0.5 N using the finger 106.
However, when a large force is applied to the LCD panel 100 with the finger 106, a local distortion occurs in the LCD panel 100, and causes display unevenness on the display screen. In particular, when a totally black display is produced, a portion, in which a distortion occurs, looks whitish. This display unevenness is serious matter for a display device.